1. Field of the Invention
This invention relates to optical coating apparatus and, in particular, to apparatus for evaporating a plurality of layers of various substances upon a substrate, utilizing an optical coating witness chip changer and a heater. Accordingly, it is a general object of this invention to provide new and improved apparatus of such character.
2. General Background
The apparatus described herein is utilized for making multilayer coatings for optical devices. The apparatus provides the operator with a means of remotely changing witness chips that are used to monitor the optical coatings and utilizes an improved heater for maintaining substrate and witness chip changer temperature.
Optical coatings are traditionally applied by resistance or electron beam evaporation techniques at elevated temperatures and in a vacuum. Because optical coatings characteristics are particularly sensitive to thickness, and the rate of application is affected by numerous variables, such as temperature, contaminants in the vacuum system, sweep rate of application or characteristics of the electron gun, it is desirable to monitor the coating characteristics as coating is applied. It is known that a reliable technique is to optically monitor the interference characteristics of the coating as it is applied to a witness chip. Most optical coatings require continuous application of multiple layers and several materials. Thus, to provide a fresh monitoring surface, frequent changes of the witness chip are desirable during the application of a particular coating. As coatings are applied in layers of 1/4, 1/2, 3/4 or full wavelength, and some cases an 1/8 of a wavelength, a new witness chip for each layer is highly desirable.
The requirements for a witness chip changer/heater, wherein a substrate may be coated with 100 layers of material depending upon the end usage, are as follows:
1. Because vacuum should not be released during application of successive layers to avoid danger of contamination to the coating, the changer should be reliably operated remote to the vacuum system.
2. Because the optical characteristics of many coatings vary significantly with temperature at the time of application, the witness chip should be heated to the same temperature as the substrates.
3. Coating characteristics vary with distance. Thus, the witness chip should be positioned approximately the same distance from the source as the substrates.
4. In order to provide access for changing and removing substrates, the changer/heater should be easily removable.
An optical device such as a blocking filter may utilize thirty layers of material to provide a 10 nm wide bandwidth. Blocking filters can be used to block various ranges of radiation ranging from infrared to X-rays.
Numerous devices have been developed by others to provide witness chip changing capability to optical coating systems. The most common type of witness chip changer utilizes cassettes, which are generally unreliable, complicated, and often a poor match up to the substrate/changer heating system and being incompatible with current vacuum system technology.
Substrate and witness window heating is most commonly accomplished by either quartz or resistance radiant heaters. Quartz radiant heaters by their nature are bulky, provide uneven heating, interfere mechanically with mechanisms and tend to do significant thermal damage to mechanical system components. Resistance heaters have a tendency to fail to provide uniform substrate heating.